The square-four engine design is a hallmark of innovation and performance in motorcycle engineering. This design, which arranges four cylinders in a square configuration as closely together as possible, is technically a “U” engine design and has been utilized by a few manufacturers seeking to maximize packaging efficiency while maintaining peak power delivery.

Ariel Square Four: Pioneering the Concept

British manufacturer Ariel kickstarted the square-four engine journey. The Square Four motorcycle was added to the Ariel lineup in 1931, courtesy of designer Edward Turner. It had two counter-rotating crankshafts and a camshaft located centrally, operating eight overhead valves. Originally fitted with a chain-driven overhead camshaft engine displacing 500cc, it had grown to incorporate a 1,000cc overhead valve unit by 1937 developing around 40 horsepower. Despite its new design, the Ariel Square Four had inherent design problems due to its air-cooled nature — including overheating in the rear cylinders — that prevented it from being used much more widely.

The 1949 Ariel Square Four engine produced approximately 35 horsepower.

Suzuki’s Foray into Square-Four Engines

Suzuki went into square-four engine design territory in that decade with their racing motorcycles of the 1960s. Produced back in 1967, the RS67 was powered by a 125cc square-four two-stroke engine. The crankcase temperatures were too high for the engine to succeed, however. Not to be dissuaded, Suzuki let loose the RG500 Gamma in the 1980s, first as a  Grand Prix competition machine, and later as a street-legal motorcycle. It had a 500cc square-four two-stroke engine with rotary valves that produced around 95 horsepower at 9,000 RPM. With over 15,000 units produced, it represented a success for square-four technology.

Kawasaki’s KR500

Kawasaki later dabbled with the square-four configuration in the late ’70s with their KR500. Its 500cc square-four two-stroke was tilted at a 45-degree angle and used rotary valves. Developing around 120 horsepower, the engine allowed for top speeds of more than 290 km/h. Even with its power output, it was ultimately difficult to be used in the KR500 and it never quite matched similar output machines from Honda or Yamaha.

The Suzuki RG500 Gamma street bike’s engine was almost identical to the MotoGP variant from the previous half-decade. This was the most recent attempt at a production square-four engine.

Morbidelli 500

Italian engineer Giancarlo Morbidelli made his mark with the development of a 500cc square-four motorcycle during the 1970s. This bike featured an aluminum monocoque frame that integrated the fuel tank and engine into a single structure, showcasing innovative design and engineering. Morbidelli’s creations earned him four Grand Prix championship titles, adding a notable chapter to Italian motorcycle history.

Kawasaki’s Unreleased 750cc Square-Four

In the early 1970s, Kawasaki embarked on developing a 750cc square-four two-stroke engine aimed at producing the fastest road bike of its time. The design reduced the frontal area compared to inline engines and featured a two-in-one exhaust system with fuel injection. However, the project was canceled in August 1973 due to impending strict emission regulations, and the bike never reached production.

The square-four engine configuration, while offering advantages in compactness and power delivery, presented challenges such as cooling issues and mechanical complexity. These factors limited its widespread adoption. Nonetheless, the square-four remains a fascinating chapter in the history of motorcycle engineering, reflecting the relentless pursuit of innovation and performance by manufacturers and engineers alike.